Brushless double-fed generator
Figure 1 shows the schematic diagram of the brushless doubly-fed generator. Two sets of independent three-phase windings are placed in the stator core slots of the generator. One set of windings is directly connected to the grid and is a power winding. The outlet terminal is denoted by A, B, C, and the number of pole pairs is p; The converter is connected to the power grid and is the control winding. The outlet terminal is represented by a, b, c, and the number of pole pairs is pc. There is no direct electromagnetic coupling between the two sets of windings, but indirect coupling through the magnetic field modulation of the rotor to achieve energy conversion. Generator rotors are divided into two categories: cage rotors and reluctance rotors. The rotor in Figure 1 is a concentric short-circuit cage winding structure. Cage rotors can be divided into independent concentric cage rotors and independent concentric cage rotors with common end rings according to different winding connection rules. As shown in Figure 2, two schematic diagrams of winding connections and three-dimensional effects are given. Reluctance rotors can be divided into salient pole reluctance type, axial laminated reluctance type and radial laminated reluctance type. Figure 3 shows a schematic diagram of a radial laminated reluctance rotor.
Brushless doubly-fed generators have some notable features:
First of all, when the rotor speed of the brushless doubly-fed generator changes, by adjusting the excitation current frequency in the control winding, the output current frequency of the generator power winding can be kept unchanged at 50 Hz, realizing variable speed and constant frequency operation; secondly, the brushless doubly-fed generator has a simple structure, no brushes and slip rings, does not require frequent maintenance, is safe and reliable, and is suitable for wind power generation systems with harsh operating environments; thirdly, the power electronic power converter of the brushless doubly-fed generator only supplies power to the control winding, and the required capacity is small. The power winding is directly connected to the grid and the cost is low; finally, the brushless doubly-fed generator relies on the magnetic field modulation of the rotor to realize energy conversion. The generator’s power density, output current waveform, power factor and other properties still have much room for improvement, which is worthy of further study.
With the continuous increase of the single-unit capacity of wind turbines, power electronic power converters have become a bottleneck restricting the increase in system capacity. In order to break through the limitation of the capacity of power electronic devices, multi-phase wind turbines have been proposed. One of the technical solutions is to directly use a multi-phase power converter corresponding to a multi-phase generator, for example, a six-phase power converter is equipped with a six-phase generator. Another technical solution is to design the generator as a multi-group three-phase system, and configure multiple three-phase power converters accordingly to form a multi-phase multi-channel wind power generation system. Both solutions can reduce the capacity of each phase of the power converter, but in comparison. The technology of the multi-phase power converter in the former scheme is relatively difficult and not mature enough, while the technology of multiple three-phase power converters in the latter scheme is quite mature, so it has been applied in large-capacity direct-drive permanent magnet wind power generation systems. Figure 4 shows the parallel dual PWM inverter topology of a direct drive wind power generation system. The generator is six-phase, divided into two groups of three-phase, equipped with two sets of three-phase power converters to form two independent three-phase systems running in parallel.
In addition to the more common six-phase dual-channel systems, some scholars have proposed more phases of wind power generation systems, such as nine-phase permanent magnet synchronous generators and twelve-phase permanent magnet brushless DC generators. According to the number of generator phases, multiple groups of three-phase systems can be formed, with multiple channels running in parallel, as shown in Figure 5.
In addition to increasing the capacity, the use of multi-phase generators can also improve the reliability and operating efficiency of the wind power generation system. In Figure 5, if one-phase winding or power converter in a certain group fails, it can be based on different fault conditions, or all three-phase systems in the group can be removed, and the remaining healthy three-phase windings will still work normally; or only the faulty phase is disconnected, and other healthy windings are kept in normal operation, so as to avoid the phenomenon that the wind power generation system fails to cause the entire unit to shut down. This is more meaningful for offshore wind farms, because offshore wind turbines are inconvenient to maintain due to environmental factors. Once a wind turbine fails, it may be shut down for several days or even dozens of days, which seriously affects the efficiency of wind energy utilization. The use of a multi-phase multi-channel system can only cut off the faulty phase or group, while maintaining the normal power generation of other healthy phases.
In summary, the generator adopts a multi-phase winding (or multiple sets of three-phase winding) structure, which can reduce the capacity of the power converter, reduce the rated parameters of power electronic devices, save costs, and improve the redundancy and reliability of the wind power generation system .